1. Field of the Invention
This invention relates to digital television signal processing apparatus and more particularly, but not exclusively, to digital television signal processing apparatus for use with digital video tape recorders (DVTRs).
2. Description of the Prior Art
In a DVTR, an incoming television signal to be recorded is sampled, the samples are pulse code modulation coded into digital form, and the resulting digital data signals are recorded on a magnetic tape and subsequently reproduced. The reproduced digital data signals are decoded, and the decoded signals are used to form an analog signal corresponding to the original television signal.
If errors occur in the handling of the digital signals, for example due to noise or tape drop-out occuring in the DVTR, the digital signals are corrupted and then the reformed analog television signal does not correspond exactly to the original analog television signal, and the resulting television picture is degraded.
There are two main approaches to dealing with errors in digital television signals. The first approach is correction, which involves the production and use of additional data signals purely for the purposes of error detection and correction, these additional data signals otherwise being redundant. While correction provides good results, in cannot generally by used as the sole means of dealing with errors, because a comprehensive correction capability would require an excessive amount of additional data which might overload the data handling parts or raise the data rate to an unacceptable level. The second approach is concealment. This comprises the replacement of corrupted data signals by data signals generated using available uncorrupted data signals. This method relies largely for accuracy on the strong correlation that exists in the television signal.
When a DVTR is reproducing in the normal reproducing mode, that is at the normal replay speed, the data is being recovered from the tape at the same overall rate that it is required for display on television receiver. It is only necessary to provide time-base correction to synchronize the data reproduced from the tape with stable output synchronizing signals.
However, reproduction at non-normal speeds is now commonly required. Such reproduction is referred to generally as special reproduction modes and these include still, slow motion, reverse motion and the so-called shuttle mode which comprises the whole range of speeds from fast-forward to fast-reverse and which is of particular utility in searching, for example while editing. In the special reproduction modes the tape transport speed may be changed very substantially while the head drum of the DVTR is generally rotated at the same speed or at least at approximately the same speed as that used for the normal reproduction mode. The range of speeds is so great that even if dynamic tracking is used there is inevitably some mistracking, at least at some speeds in some special reproduction modes, but as much data as possible is recovered. The problems involved in handling this reproduced data, relating as it does to parts of fields and containing noise and errors, as well as needing possibly very substantial time-base correction, is taken up in our co-pending UK patent application No. 8113802 (Ser. No. 2,098,031, EP-A-0065365) which corresponds to U.S. Pat. No. 4,558,377.
The above application provides a field store arrangement formed by three field stores each of which is capable of storing digital signals relating to one field of a television signal. A digital television signal reproduced at a variable rate from a DVTR is supplied to one or to two of the field stores for writing therein, depending on the rate of supply of the digital television signal. The stored digital television signal is read from one or two of the field stores, again depending on the rate of supply of the television signal, no field store being written into at the same time that it is being read from, and the writing and reading being rotated around the three field stores cyclically.
This prior arrangement has been found to operate well, the reproduced pictures obtained in the slow motion mode being particularly good and the reproduced pictures obtained in other special reproduction modes being acceptable. However, there are still some residual problems. In particular, the arrangement is somewhat complex, and in part this is due to the fact that there is significant circuit duplication, for example in that error correction is provided for each field store and there are respective read address generators. Moreover, different synchronizing signals are used in recording and reproduction and this makes insert editing very difficult. Also there is a tendency for the reproduced picture to "break-up" at the transition between different tape speeds, and patterned effects may occur at certain tape speeds depending on the shuffling system used.